This invention relates to machine tools, and more particularly to a device for chucking in a machine-tool spindle a workpiece or a bar of material to be machined, of the type having a chuck and a socket for gripping the chuck, these elements being associated with a spindle body driven rotatingly.
Present-day automatic lathes operate at spindle speed of several thousand revolutions per minute. Moreover, in view of the material-removing capacity of modern tools, the power brought into play during machining attain much greater values than in the past. As a result, the torque which must be transmitted to the bar by the chuck gripping it in the spindle is so great that earlier chucking means, especially those of the mechanical type actuated by a cylinder cam, can no longer be used. One of the shortcomings of these previous systems is that the chucking force must be transmitted via ball bearings; and owing to the requirements mentioned above in connection with the power to be transmitted, this presents difficulties in modern machines.
It has already been proposed to use a mixed pneumatic-hydraulic transmission to grip a chuck of a workpiece holder in a machine tool (Swiss Pat. No. 563,828). A large-diameter primary piston actuated by compressed air has a small-diameter extension which acts in a hydraulic chamber, a secondary part of which is bounded by a secondary piston which is thus subjected to a multiplied force. The contemplated arrangement comprises a compressed-air control of chucking and dechucking, which can require relatively high air pressures. Furthermore, a way in which this prior art system could be applied to a rotary work spindle, especially a spindle rotating at high speed, is not disclosed in this specification.
U.S. Pat. No. 3,643,969 teaches, for a lathe spindle, a collet operated by means of a plunger displaced axially by compressed-air pressure against the bias of a gripping spring. As the control sleeve of the collet rotates with spindle, whereas the plunger moves only axially, the operation involves friction between rotating surfaces and fixed surfaces, and this is incompatible with the maximum speeds of present-day lathes.
It is therefore an object of this invention to provide an improved chucking device which is adapted to the requirements of modern automatic lathes.
To this end, the chucking device according to the present invention, of the type initially mentioned, further comprises, associated with the spindle body, a primary piston actuated pneumatically, a secondary piston actuated by displacement of a hydraulic volume, and a chucking spring, the secondary piston acting against the bias of the spring with multiplication of the force and reduction of the path owing to the action of the primary piston, and the latter being controlled from a fixed source through a passage having gauged clearance.